1. Field of the Invention
The present invention relates to a pattern forming method suitable for a lithography process and a bilayer film for forming patterns. The pattern forming method comprises a process of forming a coating, which comprises a radiation-sensitive resin composition containing an acid-dissociable group-containing polysiloxane, onto a film with a high carbon content and irradiating the coating with light such as deep ultraviolet rays, electron beams, and X-rays.
2. Description of the Background Art
A recent strong demand for high density and highly integrated LSIs accelerates miniaturization of wiring patterns.
Using short wave rays in a lithographic process is one method for miniaturizing wiring patterns. In recent years, deep ultraviolet rays typified by a KrF excimer laser (wavelength: 248 nm) or an ArF excimer laser (wavelength: 193 nm), electron beams, Xrays, and the like are being used in place of ultraviolet rays such as g-line (wavelength: 436 nm) and i-line (wavelength: 365 nm).
Novolac resins, poly(vinylphenol), and the like have been used as resin components for conventional resist compositions. However, since these resins exhibit strong absorbance at a wavelength of 193 nm due to inclusion of aromatic rings in the structure, a lithographic process by an ArF excimer laser using these resins cannot provide high accuracy corresponding to high photosensitivity, high resolution, and a high aspect ratio.
Therefore, a resin for use in a resist, transparent to a wavelength of 193 nm or less, particularly to a wavelength of 193 nm and 157 nm, and exhibiting excellent dry etching resistance equivalent to or better than aromatic rings has been desired. A polysiloxane is one such a polymer. R. R. Kunz et al. of the MIT have reported their research results showing excellent transparency of a polysiloxane at a wavelength of 193 nm or less, particularly at 157 nm, describing superiority of this polymer as a resist in a lithographic process using radiation with a wavelength of 193 nmor less (J. Photopolym. Sci. Technol., Vol. 12, No. 4, 1999). Moreover, polysiloxanes are known to exhibit excellent dry etching resistance. In particular, a resist containing polyorganosilsesquioxane having a ladder structure is known to possess high plasma resistance.
Several resist materials using a siloxane polymer have also been reported. For example, Japanese Patent Publication Laid-open No. 323611/1993 discloses a radiation-sensitive resin composition comprising a polysiloxane having an acid-dissociable group such as a carboxylic acid ester group, phenol ether group, etc., on the side chain, bonded to a silicon atom via one or more carbon atoms. Since this polysiloxane is a homopolymer, resolution cannot be increased if the acid-dissociable carboxylic acid ester group on the side chain does not efficiently dissociate. If a large number of acid-dissociable groups dissociate, on the other hand, the curing shrinkage stress of the resist film increases, causing cracks and peels in the resist film.
Japanese Patent Application Laid-open No. 160623/1996 discloses a positive tone resist using a polymer in which the carboxyl group of poly(2-carboxyethylsiloxane) is protected with an acid-dissociable group such as a t-butyl group. Since this resist protects the carboxyl groups only insufficiently, it is difficult to develop the resist containing a large amount of carboxylic acid components remaining in the non-exposed area using a common alkaline developing solution.
Japanese Patent Application Laid-open No. 60733/1999 discloses a resist resin composition containing a polyorganosilsesquioxane having an acid-dissociable ester group. This polyorganosilsesquioxane is prepared by the addition reaction of an acid-dissociable group-containing (meth)acryl monomer to a condensation product of vinyl trialkoxysilane, γ-methacryloxypropyltrialkoxysilane, or the like. The resin has a problem of insufficient transparency to light with a wavelength of 193 nm or less due to unsaturated groups remaining on the polymer side chains. The patent specification also describes a resist resin composition containing a polymer made by the esterification of polyhydroxycarbonylethylsilsesquioxane with t-butyl alcohol. This polymer also has the same problem as a resist as encountered by the polymer disclosed in Japanese Patent Application Laid-open No. 160623/1996 due to a low degree of carboxyl group protection.
In view of this situation, the inventors of the present invention have conducted extensive studies on resist materials containing a siloxane polymer exhibiting high transparency to light with a wavelength of 193 nm or less. However, the effect of standing waves that are considered to be caused by high transparency of this type of resists to light with a short wavelength cannot be ignored. The high transparency to short wavelength light was found to hinder formation of fine patterns.
One way of controlling the effect of standing waves is to provide an under layer film which can not only lessen the effect of standing waves, but also exhibits sufficient dry etching resistance.
To provide the under layer film with dry etching resistance, it is necessary to reduce the content of elements with high reactivity to the etching gas, such as a hydrogen atom and nitrogen atom, in the polymer composing the under layer film and to increase the content of carbon atom having relatively low reactivity to the etching gas. Conventionally, i-line resists made from a novolac resin have been mainly used for this purpose.
However, i-line resists do not necessarily possess satisfactory dry etching resistance. The resist is significantly eroded when the under layer film for silicon-type oxide films is processed. Reduction of film thickness has been unavoidable. For this reason, a thicker under layer film is used at present. However, since the pattern size is very small in the process with a minute silicon-type oxide film using a KrF excimer laser, an ArF excimer laser, or an F2 excimer laser (wavelength: 157 nm), this method of increasing the thickness of the under layer film inevitably increases the aspect ratio of patterns. As a result, the pattern is easily eroded from the sidewall by etching gas, dwindling away or collapsing. Precise pattern transfer is difficult.
An object of the present invention is to provide a pattern forming method capable of forming minute patterns without being affected by standing waves comprising the combined use of an under layer film with excellent dry etching resistance available as a thin film and a specific polysiloxane-type radiation-sensitive resin composition exhibiting high transparency to light with a wavelength of 193 nm or less, particularly, wavelength of 193 nm and 157 nm, and also capable of forming resist patterns with a high aspect ratio by suitably selecting a specific etching gas in the dry etching process, and also to provide a bilayer film.